Pope Francis' decision to shun a major security detail for his visit to Brazil exemplifies his view of what the Catholic Church should be doing, his staff say.

The plan is simple: Go out into the streets. Spread the faith. Recapture the dynamism that other denominations have been using to snap up souls.

Upon his arrival in Rio de Janeiro this week, that philosophy helped produce a defining vignette of his young papacy; the pope rolling down the window to touch the adoring crowds who surrounded his Fiat as his driver and bodyguards struggled to get him on his way.

His call for a more missionary church, seeking out the faithful in the most marginal of places, will get even more traction on Thursday (local time) when he visits one of Rio's shantytowns, or favelas, and meets a family inside their home.

But while his subordinates in the Roman Catholic Church may appreciate that message, many are uneasy about the lengths he seems willing to go to deliver it.

''He's used that phrase that we have to get out to the streets, we can't stay locked up in our sacristies, we can't be navel-gazing all the time,'' US Cardinal Timothy Dolan said.

Dolan, however, expressed concern over Monday's swarm and said security might need to be tightened for Francis' own good.

''I love him and I don't want another conclave. We just finished one so we don't need him to be hurt at all,'' Dolan said.

Francis' car was mobbed after the lead vehicle in his motorcade made a wrong turn and got blocked by buses and taxis, enabling tens of thousands of frenzied Brazilians to surround him. But even along the planned route, there were few fences and no uniformed police or armed forces, as would be expected for a visiting head of state.

Just a few dozen plain-clothed Vatican and Brazilian security forces trotted alongside Francis' car, at times unable to keep the crowds at bay.

On Tuesday, Brazilian security officials defended their handling of the pope's tour through Rio, saying an evaluation of his arrival by federal police, the mayor's office and highway police was ''positive, since there was no incident involving the pope or with any of the faithful.''

Authorities in Brazil said earlier that about 10,000 police officers and more than 14,000 soldiers would take part in the overall papal security plan, but on Monday virtually no uniformed officers were seen.

Andreas Widmer, a former Swiss Guard who protected Pope John Paul II from 1986-1988, said the scenes from Rio were reminiscent of some of the more hair-raising trips John Paul took, even after he was wounded in a 1981 assassination attempt in St Peter's Square. He sees it as part of the pontiff's job.

''Fundamentally one has to see that the pope is not like a president,'' Widmer said. ''You can shut the president in a house and he never sees any normal people. The pope's office is a ministry, and a ministry cannot be impeded by security.''

He added, ''You cannot be pope and not see people.''

Sao Paulo Cardinal Odilo Scherer said that ''nothing happened when the pope was stuck in traffic'' and that ''we shouldn't exaggerate the psychosis of security'' when it comes to protecting the pope.

It was Francis' wish that his security not be ''militarised,'' according to Vatican spokesman, the Rev Federico Lombardi.

Francis stopped to kiss babies and shake hands thrust into the window of his car, and once he reached Rio's centre, he switched to his open-air vehicle and drove right back into the crowds. The moment was particularly unnerving in light of sometimes violent anti-government protests that have been going on across Brazil for a month.

It also was embarrassing for security officials who are charged with keeping order during next year's World Cup and the 2016 Olympics.

''I was so surprised!'' said the Rev Joseph Tan, a priest from the Philippines who echoed the reaction of many in Rio for the papal visit. ''In the Philippines, people would have gathered to get a glimpse, but nothing like what we saw,'' Tan said.

''But that's the pope's personality. He was just being himself.''

Francis was dubbed the ''slum pope'' in his native Argentina for the amount of time he spent in dangerous areas while he was archbishop there. And in a speech that some say helped get him elected pope, the former Cardinal Jorge Mario Bergoglio told colleagues that the church must ''move toward the peripheries, not only geographic but also existential.''

Francis is in Brazil for World Youth Day, a church event that takes place about every three years and brings together young Catholics from around the world. A cold rain Tuesday night didn't stop upward of 100,000 faithful from gathering on Rio's Copacabana beach to mark the event.

Clergy celebrated the opening Mass on a massive white stage covered with a bright red carpet as the crowd held aloft flags from scores of nations.